A silica (SiO2) film formed by a vacuum process such as chemical vapor deposition (CVD) generally finds wide application as an interlayer insulator film in a semiconductor device and so forth. Recently, an insulator film of a coating type based on a hydrolysate of tetraalkoxysilane and called a spin-on glass (SOG) film is also used for the purpose of forming a more uniform interlayer insulator film. In addition, an interlayer insulator film of low dielectric constant based on polyorganosiloxane called organic SOG is being developed as the packaging density of semiconductor devices and so forth is increased.
The relative dielectric constant, however, is about 4 even in the case of a CVD-SiO2 film exhibiting a lowest dielectric constant among inorganic films. The SiOF film which, as being a CVD film of low dielectric constant, is being discussed has indeed a relative dielectric constant of about 3.3 to 3.5, but is highly hygroscopic and, accordingly, will be increased in dielectric constant with use.
Under these circumstances, it has been proposed to add a solvent with a high boiling point or a pyrolytic compound to organopolysilixane as an insulator film material excellent in insulating property, heat resistance and durability, so as to form pores in an insulator film and thereby reduce the dielectric constant of the film. A film made porous as above, however, is also reduced in mechanical strength, and its dielectric constant is increased due to the moisture absorbed. Moreover, copper used for wiring may be diffused in the insulator film because the pores formed in the film are communicating with one another.
Also known is an attempt to obtain a film with a low refractive index and a low density by the application of a solution prepared by adding a cage compound of low molecular weight to an organic polymer (see Patent Literature 1). The film which is obtained by the addition of a cage compound monomer, however, is not necessarily satisfactory in such properties as dielectric constant and Young's modulus of elasticity from a practical viewpoint, and is disadvantageous in that the surface state of a coating film is deteriorated, and the film thickness considerably decreases during baking.